Thermographic evaluation typically involves irradiating an object with infrared radiation, and then analyzing the heat reflected from the object to determine a particular characteristic of the object. Prior art thermographic evaluation techniques disclosed in U.S. Pat. Nos. 4,733,079 and 4,795,906 provide a method and apparatus for thermographically identifying parts. Identification codes are etched into the parts. Infrared radiation is directed at the parts, and the reflected radiation is analyzed using image processing hardware to identify the code numbers etched into the parts. U.S. Pat. Nos. 4,854,724 and 4,647,220 respectively disclose thermographic techniques for evaluating defects in spot welds and corrosion in metallic surface structures. The evaluation techniques of the aforementioned patents include the application of heat from a pulsating infrared source, scanning the heated surface of the object with a sensing device to determine temperature differentials, and generating an image of the object from the temperature differentials. The resulting images provide an indication whether there are any defects in the spot weld or whether the object is corroded.
These prior art thermographic nondestructive evaluation techniques depend on incremental temperature variations which are detected on the surface of the material under evaluation. The prior art is limited by a minimum resolvable temperature difference (MRTD) of approximately 0.1.degree. C. In other words, a defect must cause a temperature difference of 0.1.degree. C. to be detectable, and many small defects, therefore, go undetected. If the test sample material conducts heat at a high rate, then a large amount of heat input is also required. The local heat rise caused by the defect, however, can often cause the defect to be masked by the heat input to the surface. Once the surface is heated over a few degrees, all but the largest defects will go undetected. Materials which have a low rate of heat transfer, such as most composites materials, compound this problem and reach surface saturation faster, and consequently such materials are more difficult to evaluate especially when evaluating small defects or flaws.
Accordingly, there is a need for a thermographic evaluation technique that can overcome the problems associated with the prior art techniques, that requires less heat and that detects smaller defects or flaws.